At high enough CO2 levels, clouds will start to physically break apart

Stratocumulus clouds are rather boring. They’re not as elegant as cirrus clouds (those horsetail wisps high in the sky) or as majestic as cumulonimbus clouds (big, scary thunderheads).

But stratocumulus clouds, which hover low in the sky and create vast decks of cloud cover, have a supreme value in our warming world: Their white tops reflect lots of solar radiation back into space.

But Earth’s broad portfolio of clouds in the year 2019 could potentially be altered by extreme climate change. Those stratocumulus cloud decks could vanish, further intensifying global warming.

That’s the unsettling conclusion of a study published Monday in the journal Nature Geoscience, based on a computer model that provides a new warning that climate change could deliver surprises on top of the already existing and clearly predictable consequences.

The lead researcher, Tapio Schneider, a climate scientist at Caltech, hypothesized that very high levels of atmospheric carbon dioxide could suppress the formation of stratocumulus cloud decks. He and his colleagues modeled the formation of such clouds and, after two years of computer calculations, concluded that the steady rise in atmospheric CO2 could trigger a sudden spike in temperature associated with disappearing stratocumulus clouds.

The effect appeared intense if CO2 reached 1,200 parts per million – three times the current level, which is already much higher than the preindustrial level of carbon dioxide. If CO2 reached 1,300 parts per million, the new report states, the global atmospheric temperature would rise 8 degrees Celsius (46 degrees Fahrenheit) above whatever warming had already been produced from greenhouse gases.

“Once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred,” according to the study.

Kerry Emanuel, a professor of atmospheric science at MIT, said of Schneider’s study: “What he’s done is certainly plausible, but these clouds are really hard to simulate. . . . It provides a plausible, but not yet proven, route by which you could have a tipping point in the climate.”

Climate scientists have long been confounded by clouds. A cloud can amplify global warming, or it can limit it, depending on what kind of cloud it is, and its size, location, thickness, duration, etc. But clouds are hard to pin down in a computer model.

They are remarkably insubstantial elements of the natural world. If you could bring all the clouds and water vapor in the atmosphere to the surface, it would form a liquid layer less than an inch deep, Schneider said, and clouds alone would create a layer no deeper than a coat of paint.

“You need to predict what small fraction of that water vapor will condense into clouds,” Schneider said.

There is no easy way to test whether clouds would really behave this way in a world with such alarmingly high concentrations of carbon dioxide. What’s certain is that a spike of 8 degrees C, in addition to warming already baked in the cake from greenhouse gas emissions, would presumably be catastrophic, not only for human civilization but for countless species and ecosystems jolted by the rapid climate change.

Since the start of the Industrial Revolution, when people began burning fossil fuels on a vast scale, global temperatures have risen about 1 degree Celsius, or about 1.8 degrees Fahrenheit, with the warming driven by the rise in atmospheric carbon dioxide, from about 280 ppm to more than 400 ppm, a level surpassed in 2013 for the first time in recorded history.

It is hard to imagine a world with anything near 1,300 ppm CO2.

Schneider, for one, does not think such extreme levels of CO2 will actually materialize, simply because he assumes that human civilization will find a way to avoid putting all that carbon into the atmosphere.

“I hope there’s going to be sufficient technological progress that we’re not going to get there. But it’s not outside the realm of the possible,” Schneider said.

Matt Huber, a Purdue climate scientist who has studied the effects of clouds on climate, offered a cautious assessment of the new paper.

“Whenever you see a surprising result in a climate model, you get concerned that the model is itself just too tippy, that there’s something that should be stabilizing the model,” he said.

But Huber noted that the Schneider paper offers a potential answer to a long-standing riddle. For decades, scientists have known that 55 million years ago, the Earth endured a strangely hot phase – called the Paleocene Eocene Thermal Maximum (PETM). That’s the famous “crocodiles in the Arctic” period.

How did Earth get so sweltering? Carbon dioxide is an obvious element of the mystery, but climate models cannot seem to nudge the planet into such high temperatures without extraordinary levels of CO2, such as 4,000 ppm or higher. And the geological record does not show CO2 higher than 2,000 ppm.

So there must be another factor.

One possibility has been that a massive escape of methane from the ocean floor tipped the climate into a new hothouse regime. But the Schneider paper offers another conjecture: Vanishing cloud cover could lead to a climate tipping point.

Stratocumulus clouds are produced as warm air rises from the surface and cools, causing water vapor to condense. Such cloud decks are known in California as marine layers, and they are notorious for rolling into coastal cities and turning warm days cold.

These clouds cover large swaths of the tropical ocean.

The formation of the cloud decks depends on the cooling process at the top of the clouds. This occurs at a physical scale that traditional climate models cannot easily capture.

“They simply fall through the computational mesh,” Schneider said.

He and his colleagues have developed a new model that uses what is known as a large eddy simulation. The model indicates that the cooling process necessary for these cloud decks will be suppressed if the planet gets too warm.